1. Field of the Invention
The invention relates to a protective coating for a metal component, said coating having good corrosion resistance in a saline atmosphere, to a method of forming such a coating, and to metal components having said coating. The invention is applicable in particular to the protection of steel aeronautical components, such as the components of aircraft engines, which require a high degree of reliability, and to the protection of aluminum-alloy components precoated with a chemical zincate sublayer.
2. Summary of the Prior Art
To protect steel components from saline corrosion, it is known to use cadmium, deposited electrolytically, as a protective anodic coating. This coating can be used hot, up to temperatures of about 235.degree. C.
Although cadmium provides metal components with good corrosion protection, it does have a high degree of toxicity and exhibits intrinsic incompatibilities when used with some materials. In particular, cadmium possesses a risk of intergranular corrosion, with the formation of cracks, when in contact with titanium and its alloys, and unfavorable catalytic action when in contact with synthetic oils and with fuels.
Various types of coatings have been proposed to replace cadmium. In particular, zinc-nickel coatings containing from 6% to 8% of nickel and produced in a noncyanide-containing alkaline medium have proved beneficial, since they provide good resistance to saline corrosion, but their behavior in alternating cycling is poor.
It is also known that, in the connector field, tin-nickel coatings containing 35% of nickel and deposited on a copper sublayer provide good corrosion-resistance properties. However, this type of coating does not behave in a sacrificial manner with respect to steel substrates, thereby limiting its lifetime under harsh conditions, such as alternating cycling.